1. Field of the Invention
The invention concerns matrices of photosensitive elements.
The standard way to make matrices of photosensitive elements is to provide for a network of conductors in rows and a network of conductors in columns with a respective photosensitive element at each intersection of a row and a column. Through the network of rows, a row of photosensitive elements is selected, for which it is desired to know the electrical output signals. Through the network of column conductors, a respective output signal is read for each of the elements of the selected row.
2. Description of the Prior Art
The French patent application No. 86 00716 describes a photosensitive matrix having a network of photosensitive dots wherein each photosensitive dot has a charge storage capacitor in series with a photosensitive element, this unit being connected between a row conductor and a column conductor. The photosensitive element may be an PIN photodiode with three layers (a P type semiconducting layer, an intrinsic layer I and an N type layer).
Charges are generated in the photodiode by the illumination of the photosensitive dot. These charges get accumulated at the (floating) node between the photodiode and the capacitor. They are read by the application, to the row conductor, of a voltage pulse in one direction which gives the photodiode a forward bias (whereas it was reverse biased during the accumulation stage). Charges, of a quantity corresponding to the accumulated charges, are then transferred from the floating node to the column (or conversely from the column to the floating node). The reading operation consists in measuring this movement of charges.
After the end of the reading pulse, the photodiode turns off for a new stage of illumination and integration of charges.
But the potential of the floating node no longer has the value that it had at the start of the integration stage. It is therefore not possible to begin a new integration stage without setting up this floating node potential at a well-determined starting value.
The reading stage is therefore followed by a stage for resetting the potential of the floating node.
The resetting is done by intense illumination of the photodiode. It is therefore necessary to provide for a source of intense illumination and for control means synchronized with the photosensitive matrice reading means, in order to perform a resetting operation after each reading stage.
Furthermore, the above-described matrices, in which a unit in series with a photodiode and a capacitor is used as a photosensitive dot, require that illumination should be discontinuous, namely, the matrix should be in darkness during the time reserved for the reading of the signal given by the photosensitive dots.
The present invention proposes a new photosensitive dot structure which removes the need for a source of illumination for resetting, and which enables the acquisition of information in a continuous flow without its being necessary to place the matrix in darkness during the reading stage.